The present invention relates to a multifunctional metallic coagent that improves the properties of a rubber by crosslinking with peroxide and vulcanizing the rubber.
As a crosslinker, sulfur and sulfur complex which can be used the rubber curing stocks and the peroxide crosslinker for a long time, but the unsaturated carboxylic acid derivatives and its metallic salts are used the rubber vulcanizing agent in various field recently. Generally, crosslinking with peroxide results in the formation of a covalent bond, so the carbon-carbon bond is rigid and stable, but the cured rubber has the lower tensile and tear strength. And the good heat stability of this covalent bond explains the superior heat aged characteristics of peroxide crosslinking systems. In contrast, crosslinking with the sulfur and its derivatives are thermally weak but are mobile under stress and can slip along the hydrocarbon chain. This mobility explains the superior tensile and tear strength in the sulfur curing systems. Meanwhile, in case of the coagent of unsaturated carboxylic acid, there are so many crosslinkers to cure the rubber, the metallic salt of unsaturated carboxylic acid among them, which is crosslinking with peroxide as an initiator, the crosslink bond results in the formation of an ionic bond. This crosslink bond embodies the best characteristics of both peroxide and sulfur crosslinking systems, that showing a good heat stability and the ability to slip along the hydrocarbon chains giving the superior tensile and tear strength. The di-functional metallic coagent is employed as a metallic coagent of unsaturated carboxylic acid on the market that made from acrylic acid or methacrylic acid reacting with divalent metals such as zinc, calcium and magnesium. This di-functional metallic coagent of unsaturated carboxylic acid is a useful thing to improve the properties and curing the rubber, especially a core of golf ball in a field of elasticity. But, there is a difference between the properties of cured rubber depends on the kind of unsaturated carboxylic acid such as acrylic acid, methacrylic acid even in the metallic coagent of an unsaturated carboxylic acid. For an example, in case of the cured rubber using the metallic salt of acrylic acid that has a good elasticity, but has a defect in the strength of tensile and tear compared the cured rubber using the metallic salt of methacrylic acid. Using two kinds of metallic coagent revealed a good characteristic than using one kind of metallic coagent in some cases.
Moreover, in a shelf-life stability, there is no difference between the properties of metallic coagent of acrylic acid for a period of one year, but the metallic coagent of methacrylic acid should be used within six months for optimum product performance, after a lapse of six months, a falling-off occurred in the product performance. And, there are many difficulties to select a metallic coagent when crosslinking the rubber, because the properties are different depend on the used metal. Generally, there are many multifunctional coagents of unsaturated carboxylic acid in a phase of liquid except a crosslinker in a phase of metallic salt, that is difficult to mix with a rubber and hard to achieve a uniform performance of cured rubber due to a migration of liquid crosslinker in the mixture. In contrast, the metallic salt of unsaturated carboxylic acid in a form of powder, that is easy to get a uniform mixture when blending with a rubber and obtaining a good performance of cured rubber. Thereupon, this inventor made a new multifunctional metallic coagent of unsaturated carboxylic acid for crosslinking a rubber.
The metallic coagents according to the present invention have a good characteristic which revealed using two kinds of metallic coagent in cured rubber as mentioned above, and to solve the problems in the mixture of common multifunctional crosslinker in a phase of liquid. The unsaturated carboxylic acids which can be used for a multifunctional metallic salt of unsaturated carboxylic acid in accordance with the present invention are mono-basic acid, such as acrylic acid, methacrylic acid and di-basic acid, such as maleic acid, fumaric acid. As a mono-basic acid, one kind of unsaturated carboxylic acid can be used among acrylic acid and methacrylic acid, or two kinds of unsaturated carboxylic acid can be used among them. And, as a di-basic acid, only one kind of unsaturated carboxylic acid can be used among maleic acid and fumaric acid. Meanwhile, as a metal complex for making a metallic salt of unsaturated carboxylic acid, divalent metal oxide, such as zinc oxide, calcium oxide and magnesium oxide can be used. One kind of metal oxide or two kinds of metal oxide can be used among them. For the reaction of metal oxide and unsaturated carboxylic acid according to the present invention, it must be react in organic solvent, such as toluene, ethanol, methanol and isopropanol that can be used alone or mixed. Basically, this invention is made by reaction from three mols of metal oxide with two mols of mono-basic acid and two mols of di-basic acid, being a multifunctional metallic salt of unsaturated carboxylic acid.
For an example, using two mols of acrylic acid as mono-basic acid, two mols of maleic acid as a di-basic acid and three mols of zinc oxid as a metal oxide, the reactants which eliminated the organic solvent and water due to the reaction that being stable state, trizincdimaleicacrylate is made in a form of white powder. This reaction is represented in the following reaction formula. 
The crosslinking of generated trizincdimaleicacrylate with a rubber is as follows. Hereinafter, for the sake of convenience, cis-1,4 polybutadiene is described in the chemical reaction formula of this detail description as a rubber which expresses synthetic rubber, such as polybutadiene rubber, polyisoprene rubber, polystyrenebutadiene rubber, polyethylenebutadiene rubber, silicone rubber and etc. As mentioned above, to crosslink a rubber with organic peroxide as an initiator, reacting and crosslinking by heating in about 150xcx9c160xc2x0 C. in a proper molding machine. This multifunctional metallic salt of unsaturated carboxylic acid crosslinking a rubber with an assistance of peroxide as an initiator in the process of react, generates the crosslink bond in the formation of an ionic bond that giving a good heat stability and the superior tensile and tear strength to the cured rubber. Meanwhile, this trizincdimaleicacrylate is represented in a formula as follows. 
The crosslinking reaction of cis-1,4 polybutadiene and this multifunctional metallic coagent is represented in a formula as follows, the cured rubber having ionic bond which giving a superior tensile and tear strength. 
The abovementioned multifunctional metallic salt of unsaturated carboxylic acid is obtained from the reactant by using one kind of acrylic acid as a mono-basic acid, one kind of maleic acid as a di-basic acid and one kind of zinc oxide as a metal oxide. Meanwhile, as an other example, a multifunctional metallic salt of unsaturated carboxylic acid from the reactant by using two kinds of mono-basic acid, one kind of di-basic acid and one kind of metal oxide is as follows.
The reactant by using one mol of acrylic acid and one mol of methacrylic acid as a mono-basic acid, two mols of maleic acid as a di-basic acid and three mols of zinc oxide as a metal oxide, that can be generated trizincdimalemethacrylic-acrylate in a white powder. This reaction is represented in a formula as follows. 
The crosslinking reaction of cis-1,4 polybutadiene and this multifunctional metallic coagent with an assistance of peroxide at 155xcx9c170xc2x0 C. in a proper molding machine, the cured rubber having strong ionic bond which giving a superior tensile and tear strength in the same way as above. This cured rubber having strong ionic bond is represented in a chemical formula as follows. 
The used metal oxide of the abovementioned examples of reaction is only one kind of zinc oxide. But, if making a multifunctional metallic salt of unsaturated carboxylic acid by using two mols of zinc oxide and one mol of magnesium oxide, that is, two kinds of metal oxide, and the unsaturated carboxylic acid which comprises two mols of acrylic acid as a mono-basic acid and two mols of maleic acid as a di-basic acid, that can be generated monomagnesiumdimalezincdiacrylate in a form of white powder. This formula is as follows. 
When this multifunctional metallic salt of unsaturated carboxylic acid crosslinks to cis-1,4 polybutadiene with an assistance of peroxide, the cured rubber also having the structure of strong ionic bond that giving a good property.
Meanwhile, the organic solvent which is used in time of making of the abovementioned multifunctional metallic coagent, that should be easily control the generated heat due to a violent exothermic reaction when reacting a metal oxide and an unsaturated carboxylic acid, having an hydroxyl radical in the structure is proper to this invention. Generally, in a reaction of an unsaturated carboxylic acid and a metal oxide, the excessive reaction, in other words, the reaction is not steady and consecutive that cause the reacting heat is generated in a state of uncontrollable, that easily making an ester oligomer and regenerating a metal oxide by the aid of small amount of oxygen in the firstly mixed air. This generated ester oligomer is an oily matter and having a sticky character, that is difficult to dry the generated water and to extract the organic solvent of reaction system. Moreover, the ester oligomer mixed with the metal oxide is difficult to blend with a rubber in milling operation due to a sticky state, and could not to increase a crosslinking density and hardness of the cured rubber, that is not qualify to be a crosslinker, only acting as a filler. In a reaction of an unsaturated carboxylic acid and a metal oxide of the present invention, the ester oligomer is generated if the temperature failed to control. Therefore, when making a multifunctional metallic coagent, it is important to select a proper organic solvent in accordance with the kind of an unsaturated carboxylic acid and the metal oxide. In other words, organic solvent must be easily absorbing the reaction heat, small amount of ester oligomer and the generated water in organic solvent. Especially, ethanol and isopropanol are proper solvent to solve the problems, occasionally small amount of an inorganic adsorbent like zeolite can be used. This inorganic adsorbent assists to absorb rapidly the generated water and small amount of ester oligomer. For an example, the generation of said ester oligomer is simply represented in a reaction formula as follows. 
This reaction of an unsaturated carboxylic acid and metal oxide as showing above, when receiving the excess heat directly which occurred by an unsteady reaction and partially reaction, then the phenomena became more seriously. Accordingly, selecting a proper solvent and raising the efficiency of heat exchanger and mixer for transmitting a reactive heat, the reaction of chemicals in this invention is operated. In order to make the multifunctional metallic salt of unsaturated carboxylic acid according to the present invention, as an example of the method is as follows. Firstly, put the said organic solvent into a reactor which has a stirrer for mixing the chemicals and with a jacket for exchanging the heat, and adding two mols of di-basic acid while stirring at a speed of 100xcx9c200 r.p.m., completely dissolving the acid. And, put two mols of mono-basic acid into the solution maintaining the speed of stirrer until the chemicals being dissolved completely, then, three mols of the said metal oxide is added little by little for 20xcx9c30 min. From this time, a lot of heat is generated by the exothermic reaction, rapidly cooling the jacket for exchanging the heat. Maintaining the temperature of reacting solution below 40xc2x0 C., stirring the solution in about 60 min., then, the thick paste state of multifunctional metallic salt of unsaturated carboxylic acid is made in a form of dispersion in the said organic solvent. After that time, the temperature begin to drop, the reaction comes to an end point.
This thick paste state of reactant in a form of dispersion in the organic solvent that pour into a dryer, equipped a screw type agitator, condenser and pressure reducer, by using a proper pump. And, drying the reactant while agitating in a state of reduced pressure, then the used organic solvent and the generated water are eliminated. And after, grinding the results by using a proper milling machine such as pin mill and ball mill, the multifunctional metallic salt of unsaturated carboxylic acid in a white powder is obtained. Meanwhile, the condensed organic solvent that is contained an amount of water, which can be reused by a proper separation method.